Speed control of strip rolling mills



United States Patent Qffice Patented Aug. 19, 1969 3,462,664 SPEED CONTROL OF STRIP ROLLING MILLS George E. Lemon, 286 Lora Ave., Youngstown, Ohio 44504 Filed Apr. 26, 1967, Ser. No. 633,793 Int. Cl. H02p 5/46 US. Cl. 318-67 3 Claims ABSTRACT OF THE DISCLOSURE The following specification discloses a speed control system for use with, for example, the drive motors of a hot strip rolling mill. The magnetic amplifiers of the field strength regulating circuits of the drive motors are regulated by individual rheostats of sets of rheostats, the particular rheostat controlling the magnetic amplifier being determined by the desired speed. The switches determining which rheostat controls its associated magnetic amplifier are interlocked in such a manner that when the rheostat giving the desired percentage of the maximum motor speed for a specific exit gage is selected for one magnetic amplifier, the corresponding rheostats associated with the other magnetic amplifiers are controlling their respective magnetic amplifiers and all the motors are operating at the correct speed for the desired exit gage. The rheostats of each set are connected mechanically so that a change in the setting of one rheostat of the set produces a proportional change in the settings of the other rheostats of the set.

This invention relates to the control of strip rolling mills and more particularly to means to rapidly and accurately control and vary the speed of finishing mill stands.

In the production of strip from steel billets, there are provided several (usually four) roughing stands which receive heated billets from a furnace and which effect an initial reduction in thickness of the billet. The resulting slab or bar is then passed to the finishing mill train, which may, for example, consist of six four-high mill stands, each of which efiects a further reduction in thickness of the slab, the result being a strip of precisely controlled and uniform thickness or gage. It is readily apparent that, in order to prevent cobbles or tearing of the strip during the rolling operation, the individual stands must be run at successively higher speeds from the first or entrance stand to the last or exit stand. Further, these speeds vary with the thickness of the incoming slabs, the amount of reduction effected by each stand, and the desired exit gage. In order to produce strip having uniform physical properties it is necessary to maintain the temperature of the strip within certain limits. This is accomplished by varying the total length of time the strip is passing through the mill. Thus, if, for example, an incoming slab is somewhat cooler than desired, it is necessary to shorten the rolling time by increasing the speeds of all the stands of the rolling mill.

Presently employed speed control systems are such that the speed of each stand is adjusted independently of the remaining stands thus requiring an appreciable length of time for the adjusting of the speeds of the entire finishing mill.

It is the primary object of the present invention to provide an improved control apparatus for adjusting and regulating the speeds of the stands of a finishing mill which permits all the stands to be adjusted to approximately the desired speeds simultaneously. Another object of the invention is the provision of speed control apparatus which greatly simplifies the computations required on the part of the mill operator when changing speeds to accommodate varying conditions and requirements of entrance and exit gages, strip temperature, etc.

Fora thorough understanding of my invention reference should be had to the following specification and the accompanying drawing wherein is shown a preferred embodiment of my invention.

In the drawing:

FIGURE 1 is a graph showing the relationship of the speeds of the finishing mill stand work rolls in peripheral feet'per minute at various finish gages;

FIGURE 2 is a graph showing a modified relationship of the speeds of the finishing mill stand work rolls at various finish gages; and

FIGURE 3 is a highly schematic showing of the finishing mill stands of a hot strip rolling mill equipped with the control circuit of the present invention.

A typical strip rolling mill to which the control of my invention may be applied, as shown in FIGURE 3, consists of, for example, six mill stands (V-X) each of which has a pair of motor driven work rolls 10, 11. The roughing stands and the housings, back-up rolls, etc. of the finishing mill stands are omitted from the drawing for clarity. Strip enters the finishing mill train from the roughing stands and passes progressively through stands V through X.

Although not shown herein, typical hot strip mill installations are provided with four roughing stands preceding the finishing stands and these roughing stands effect an initial reduction in the thickness of the incoming bar or slab which is then passed to the finishing mill train where it is further reduced in thickness to produce strip of precisely controlled uniform thickness. The speed of the strip as it leaves any of the stands is equivalent to the peripheral speed of the work rolls of that stand. This is also, of course, the speed at which the strip is supplied to the next stand of the mill train. The reduction effected by this second stand accelerates the strip and thus the work rolls of this stand must rotate more rapidly than those of the previous stand. As stated previously, the speeds of the various stands must be precisely controlled so as to prevent stretching and possible tearing of the strip which occurs when the work rolls of a stand are being driven more rapidly than desired, and to prevent the formation of cobbles or loops when the work rolls are driven too slowly.

The graph of FIGURE 1 shows the relationship among the speeds of the work rolls in speed units per minute of the various stands which produce the optimum rolling conditions when rolling a number of exit or finish gages. The limitations in finish gage temperature and in the top speeds of the driving motors of the later (IX, X) stands of the mill train necessitate the slowing down of the earlier (V, VI) stands when rolling light gage strip.

An analysis of the graph reveals that, within the range of exit gages from 0 to 0.400 inch, the ratio of speed change between successive stands is constant for all stands except between stands V and VI, regardless of the gag being rolled. The device of my invention utilizes this constant ratio to simplify the changing of speed of the work rolls of the stands to meet varying conditions and requirements of entrance and exit gage, slab temperature, etc.

The motors used to drive the work rolls of the strip mill are commonly of the adjustable speed shunt wound direct current type with compensated windings. In order to maintain the torque necessary for the rolling operation it is normally desirable to maintain the armature voltage constant and increase or decrease speed by varying the field strength; a decrease in field strength resulting in an increase in speed. As is well known in the rolling mill control art, regulation of the field strength is accomplished by means of control circuits employing magnetic amplifiers 20. These magnetic amplifiers are controlled by means of rheostats. A separate motor and control are provided for each stand of the finishing mill train. In order to simplify the adjusting of the speeds of the various stands and to take advantage of the constant ratio of speed variation described above, I propose to replace the presently employed rheostats with the control 21 illustrated in FIGURE 3 and described below.

The control 21 consists of a plurality of rheostats 22-31 having their variable contact points mechanically interconnected as indicated at 33 and having independent connections to contacts S1 of switch 43. The connection 32 and the contacts S2 of switch 43 are connected to the control circuit in the same manner as are the terminals of the presently employed controls. Corresp nding contacts S21 and S22 are provided on switch 43 for connecting the rheostat elements of the control associated with the succeeding stand and likewise for the remaining stands. It should be understood that the contacts associated with corresponding rheostats are operated simultaneously. Thus, if, for example, the contacts bringing rheostat 22 into the control circuit are closed, the contacts of the next section of the switch 43 which bring rheostat 45 into its control circuit are also closed. The switch 43 is of the cancelling type, that is, if one group ofassociated contacts are closed previously closed contacts f each set are opened. Thus, only one rheostat of each control is in the circuit at any given time.

Each of the rheostats 22-31 possesses identical electrical characteristics and the mechanical interconnection 33 is such that when, in the illustrated embodiment, the variable contact of rheostat 22 is in its extreme position,

allowing the drive motor to operate at top speed, thev variable contacts of rheostats 23-31 are at positions which are 90%, 80%, etc., respectively, of their extreme positions. When the variable contact of rheostat 22 is moved to a position which is, for example, 80% of its 100% position, the variable contacts of the remaining rheostats are likewise moved to positions which are 80% of their corresponding positions. It should be understood that if, for example, the rheostat is positioned at its extreme or 100% position, the rheostats 22, 23, and 24 are all at their extreme or 100% positions. This relationship of the positions of the various rheostats holds for all positions of the rheostat 22. It will be readily apparent that, While for purposes of illustration, there are shown ten rheostats, the number of rheostats may be greater or less as determined by preference. Regardless of the number of rheostats in each set, the percentage difference between the values of the positions of successive rheostats in the control set are equivalent to the reciprocal of the number of rheostats.

Control of motor speed is accomplished by first adiusting the variable contacts of the sets of ganged rheostats 21 in such manner that the speeds of the work roll driving motors are in the above mentioned ratio and then actuating the switch 43 so as to bring the rheostats which result in the desired speed producing the correct finishing temperature into the control circuit. It will be apparent that .4 I have, in effect, separated the control of speed into two parts; the first being the relative speed as determined by the reduction effected by each stand and the second being the speeds which result in correct finishing temperature. It will be readily evident that this system effects a considerable saving in time over presently employed systems as all the mill stands may be brought to the desired speed in one operation whereas it is now necessary to control each stand separately. Due to the inertia of the motors, transmissions, and work rolls, a substantial period of time must be allowed for the rolls to reach the desired speed. In the illustrated embodiment of my invention there are provided ten rheostats in each control. Thus, the first rheostat 22 of the set may be considered to give 100% speed of the work roll drive motor; the next rheostat 23, to give speed, etc.

If meters indicating the percentage deviation of work roll peripheral speed of each stand compared with that of stand VII are provided, the computations required of the mill operator in making adjustments and corrections in speed are further simplified. This is illustrated in the graph of FIGURE 2 wherein the speed of the rolls of stand VII is indicated as a constant The advantage of this method can best be illustrated by means of an example; if it is desired to roll strip having a finish gage of 0.100 inch, the correct meter readings are 35, 60, 100, 140, 180, 200 for stands V through X, respectively. These figures are readily remembered, simpler than the actual speed figures given above, and enable the operator to readily determine if the work rolls are being driven at the proper speeds.

As the speed differences of stands V and VI and 0f stands VI and VII do not follow the uniform ratio as do the remaining stands, the control of my invent1on may, if desired, be applied only to stands VII through X and stands V and VI may be controlled as is presently done.

In addition to simplifying the speed regulation of the finishing mill train, the control of my invention facilitates the speeding up of the rolling operation which is neces' sary in circumstances where it is desired to maintain the temperature of the strip being rolled within a specified temperature range in order to insure uniform properties of the finished strip. When it is desired to accelerate the rolling operation switch 43 is actuated so as to bring the rheostat controlling the next higher percentage range into the control circuit. Since switch 43 effects this change simultaneously in all the rheostat sets and since the rheostats are set according to the desired speed ratio, the entire rolling mill is immediately accelerated to the desired speed. In presently employed systems, each stand is accelerated independently and, if the amount of acceleration is large, it is extremely difficult to adjust the mill stands rapidly and accurately enough to prevent the formation of damaging cobbles.

It will be readily apparent that while the embodiment described herein is applied to the control of hot strip rolling mills, the principles of my invention may be advantageously applied to other situations in which it is desired to control a plurality of motors operating at proportional speeds. It will be further apparent that changes and modifications may be made in the control system outlined above without departing from the scope of my invention. Reference should be had to the appended claims in determining the true scope of my invention.

Having described my invention what I claim is:

1. In a multi-stand strip rolling mill, each of said stands being driven by an electric motor, and wherein the regulation of the speeds of said motors is accomplished in part by a circuit regulating the field strengths of said motors, said circuit including magnetic amplifiers responsive to operator controlled resistance means, and wherein the speed regulatin controls of each motor are independent of the controls of the other motors; the improvement characterized in that said operator controlled resistance means comprises a plurality of sets of rheostats, one set of rheostats being provided for each field strength control circuit; means to selectively connect one of said rheostats of each set to its associated field strength regulating circuit while maintaining the remaining rheostats of said sets in an inoperative state; and means to simultaneously actuate said means to connect.

2. Motor speed control apparatus according to claim 1 further characterized in that each of said sets of rheostats has the same number of elements and in that said means to simultaneously actuate said connecting means effects the connection of the corresponding element of each set.

3. Motor speed control apparatus according to claim 2 further characterized in that the rheostats of said sets are References Cited UNITED STATES PATENTS 4/1909 Tritle 3l884 X 5/1955 Bryden 31867 X 10 ORlS L. RADER, Primary Examiner A. G. COLLINS, Assistant Examiner US. Cl. X.R.

mechanically connected, the, arrangement being such that 5 31:3...68, 31 

